Experimental study on the τ± decay matrix element

Measurement of the Dalitz plot slope parameters of the K±→π±π+π− decay

“…The described selection criterion costs about 50% of the statistics; however an appropriate MC description of the experimental conditions is more important than the sample size for the present analysis. 2 As will be shown below in the discussion leading to the results presented in Fig. 3, the MC simulation of the experimental conditions reproduces the data distributions to a level of a few parts per mille.…”

“…The distribution of the reconstructed 3π ± invariant mass of data events (before the kinematic fitting) and its comparison 1 The acceptance is not biased by the finite outer size of the DCHs due to a relatively small Q-value of the K ± → π ± π + π − decay: Q = 75.0 MeV. 2 In a different case, the charge asymmetry analysis [5] is performed with soft cuts maximizing the selected data sample, and is based on cancellation, rather than simulation, of the systematic effects.…”

“…Moreover, time variations of the above effects during the running period were simulated. A production of 6.7 × 10 9 K ± → π ± π + π − events distributed according to the matrix element (2) with PDG values of the slope parameters [1] was performed to determine the detector acceptance. A sample of 1.16 × 10 9 MC events (almost 2.5 times larger than the data sample) passes the selection.…”

“…Here m π is the charged pion mass, m K is the charged kaon mass, P i are the pion four-momenta, the indices i, j = 1, 2 correspond to the two pions of the same electric charge ("even" pions), and the index i, j = 3 to the other ("odd") pion. The experimental Dalitz plot distribution of the K ± → π ± π + π − decay has been up to now experimentally analyzed [2,3] in terms of a polynomial expansion in powers of u and v:…”

“…Among the measurements of the slope parameters performed in the past, the most precise are reported in [2] based on 1.5 × 10 6 K ± decays, and in [3] based on 0.225 × 10 6 K + decays.…”